Production of exogenous proteins by recombinant DNA techniques is used in various industries such as pharmaceutical industry and food industry. In most cases, production of recombinant proteins is carried out by introducing an expression vector comprising a nucleotide sequence encoding a protein of interest into a host, such as Escherichia coli, yeast, insect cell, plant cell, and animal cell, selecting a transformant in which the expression vector is integrated into the chromosome, and further culturing the transformed cell line under appropriate culture conditions.
However, in order to develop a host which can produce an exogenous protein efficiently, it is necessary to select a host cell having good productivity for each protein of interest, so that a further technical innovation is desired on the exogenous protein production techniques for each host.
In the bacteria systems, such as Escherichia coli, or yeast systems, different from animal cells, post-translational modifications, such as sugar chain modification, are difficult to attain in many cases and thus cause a problem in producing a protein having its activity.
Since the produced protein is subject to a post-translational modification such as phosphorylation and addition of sugar chains in the insect system, this system has a merit that the protein having its original physiological activity can be expressed. However, since the sugar chain structure of the secreted protein is different from that of mammalians-derived cells, antigenicity and the like become a problem when the protein is applied to pharmaceutical use.
In addition, since a recombinant virus is used in the insect cell system when an exogenous gene is introduced, there is a problem that its inactivation and containment of the virus are required from the viewpoint of safety.
In the animal cell system, post-translational modifications, such as phosphorylation, sugar chain addition, and folding, can be conducted to proteins derived from higher animals including human, in more similar manner to those produced in the living body. Such accurate post-translational modifications are necessary for recreating the physiological activity originally possessed by a protein in its recombinant protein, and a protein production system in which a mammalian cell is used as a host is usually applied to pharmaceutical products and the like that requires such physiological activity.
However, a protein expression system in which a mammalian cell is used as the host is generally low in productivity, and also causes a problem of the stability of introduced genes in many cases. Improvement of productivity of a protein using a mammalian culture cell as a host is not only very important in producing medicaments for treatment, diagnostic agents and the like, but also greatly contributes to research and development of them. Thus, it is urgent to develop a gene expression system which easily makes it possible to obtain a cell line of a high productivity using a mammalian culture cell, particularly Chinese hamster ovary cell (CHO cell), as the host.
A transposon is a transposable genetic element which can move from one locus to other locus on the chromosome. A transposon is a strong tool for the study on molecular biology and genetics and used for a purpose, such as mutagenesis, gene trapping, and preparation of transgenic individuals, in insects or nematode (e.g., Drosophila melanogaster or Caenorhabditis elegans) and plants. However, development of such a technique has been delayed for vertebral animals including mammalian cells.
In recent years, however, transposons which have activities also in vertebral animals have been reported, and some of them were shown to have an activity in mammalian cells, such as cell derived from mouse and human. Typical examples include transposons Tol1 (Patent Reference 1) and Tol2 (Non-patent Reference 1) which are cloned from a medaka (killifish), Sleeping Beauty reconstructed from a non-autonomous transposon existed in Onchorhynchus fish genome (Non-patent Reference 2), an artificial transposon Frog prince (Non-patent Reference 3) which is derived from frog, and a transposon piggyBac (Non-patent Reference 4) which is derived from insect.
These DNA transposons have been used for mutagenesis, gene trapping, preparation of transgenic individuals, expression of drug-resistant proteins, and the like, as a gene introduction tool for bringing a new phenotype in a genome of a mammalian cell (Non-patent References 5 to 12).
In the case of insects, a method in which an exogenous gene is introduced into silkworm chromosome using the transposon piggyBac derived from a Lepidoptera insect to express the protein encoded by said exogenous gene has been studied, and a protein production method using the above techniques was disclosed (Patent Reference 2).
However, since protein of interest is not expressed at sufficient levels and is produced in the whole body of silkworm, it causes an economical problem due to the necessity of an advanced purification technique for recovering the expressed exogenous protein in a highly purified form from the body fluid including a large amount of contaminated proteins.
In addition, an example in which a protein relating to G418 resistance is expressed in a mammalian cell using the medaka-derived transposon Tol2 (Non-patent Reference 12) is known.
As one method for efficiently screening high expression cells, attenuation of a selectable marker gene is known. As a method for attenuation, amino acid modification in a neomycin resistance gene (Non-patent References 13 and 14) and binding of a destabilization sequence in dhfr gene (Non-patent Reference 15) are known. Alternatively, it is shown that high expression cells can be obtained by using an attenuated selectable marker gene.
On the other hand, it is also shown that the number of drug-resistant cells is drastically reduced by the attenuation and that, as a result, there is a possibility of not obtaining any drug-resistant cell. Thus, creation of a method for efficiently screening high expression cells is still desired.
It is known that in protein coding genes, there is codon usage bias depending on species and that human erythropoiethin expression in a CHO cell is improved by optimizing this codon bias (Non-patent Reference 16).